Prior Art Excimer Lasers
Krypton-Fluoride (KrF) excimer lasers are currently becoming the workhorse light source for the integrated circuit lithography industry. The KrF laser produces a laser beam having a narrow-band wavelength of about 248 nm and can be used to produce integrated circuits with dimensions as small as about 180 nm. Such a KrF laser is described in U.S. Pat. No. 5,023,844 which is incorporated herein by reference. A complete description of a state-of-the art production quality KrF laser is described in U.S. patent application Ser. No. 09,041,474 now U.S. Pat. No. 5,991,324 which is also incorporated herein by reference. The Argon Fluoride (ArF) excimer laser is very similar to the KrF laser. The primary difference is the laser gas mixture and a shorter wavelength of the output beam. Basically, Argon replaces Krypton and the resulting wavelength of the output beam is about 193 nm. This permits the integrated circuit dimensions to be further reduced to about 140 nm. A typical prior art excimer laser used in the production of integrated circuits is depicted in FIG. 1. A cross-section of the laser chamber of this prior art laser is shown in FIG. 2. A pulse power system comprised of a commutator module and a compression module and powered by a high voltage power supply module provides electrical pulses to electrodes 6 located in a discharge chamber 8. Typical state-of-the-art lithography lasers are operated at a pulse rate of about 1000 Hz with pulse energies of about 10 mJ per pulse. The laser gas (for a KrF laser, about 0.1% fluorine, 1.3% krypton and the rest neon which functions as a buffer gas; for an ArF laser, about 0.1% fluorine, 3% argon and the rest neon) at about 3 to 3.5 atmospheres is circulated through the space between the electrodes at velocities of about 25 meters per second. This is done with tangential blower 10 located in the laser discharge chamber. The laser gases are cooled with a heat exchanger 11 also located in the chamber and a cold plate (not shown) mounted on the outside of the chamber. The natural bandwidth of the excimer lasers is narrowed by a line narrowing module as shown in FIG. 1. Commercial excimer laser systems are typically comprised of several modules that may be replaced quickly without disturbing the rest of the system. Principal modules include:
Laser Chamber Module PA1 Pulse Power System with: high voltage power supply module, PA1 commutation module and high voltage compression head module, PA1 Output Coupler Module PA1 Line Narrowing Module PA1 Wavelength Stabilization Module PA1 Control Module PA1 Gas Control Module
These and additional modules shown in FIG. 1 are designed for quick replacement as individual units to minimize down time to the laser when maintenance is performed.
Electrodes 6 consists of a cathode and a grounded anode. The anode is supported in this prior art embodiment near the center of the chamber. Flow is counter-clockwise in this view. Peaking capacitor 54 is charged prior to each pulse by pulse power system. During the voltage buildup on peaking capacitor 54 a high electric field is created by two preionizers 56 which produce an ion field between the electrodes and as the charge on the peaking capacitor reaches about 16,000 volts, a discharge across the electrode is generated producing the excimer laser pulse and discharging peaking capacitor 54. Following each pulse, the gas flow between the electrodes of about 2.5 cm per millisecond, created by blower 10, is sufficient to provide fresh laser gas between the electrodes in time for the next pulse occurring 1.0 millisecond later.
In a typical lithography excimer laser, a feedback control system measures the output laser energy of each pulse, determines the degree of deviation from a desired pulse energy, and then sends a signal to the control module to adjust the power supply voltage so that the energy of subsequent pulses are close to a desired energy.
These excimer lasers are typically required to operate continuously 24 hours per day, 7 days per week for several months, with only short outages for scheduled maintenance.